The present invention relates to apparatus for manufacturing of integrated circuits.
One of the basic problems in integrated circuit manufacturing is particulates. This problem is becoming more and more difficult, because of two trends in integrated circuit processing: First, as device dimensions become smaller and smaller, it is necessary to avoid the presence of smaller and smaller particles. This makes the job of making sure that a clean room is really clean increasingly difficult. For example, a clean room which is of class 1 (has one particle per cubic foot) for particles of one micron and larger may well be class 1000 or worse if particle sizes down to 100 angstroms are counted.
Second, there is increased desire to use large size integrated circuit patterns: for example, integrated circuit sizes larger than 50,000 square mils are much more commonly used now than they were five years ago.
Thus, particulates are not only an extremely improtant source of loss in integrated circuit manufacturing, but their importance will increase very rapidly in the coming years. Thus, it is an object of the present invention to provide generally applicable methods for fabricating integrated circuits which reduce the sensitivity of the process to particulate contamination.
One of the major sources of particulate contamination is human-generated, including both the particles which are released by human bodies and the particles which are stirred up by equipment operators moving around inside a semiconductor processing facility (front end). To reduce this, a general trend in the industry for several years has been to make more use of automatic transfer operations, wherein a technician can, for example, place a cassette of wafers into a machine, and then the machine automatically transfers the wafers, one by one, from the cassette through the machine (to effect the processing steps necessary) and back to the cassette, without the technician's having to touch the wafers.
However, the efforts in this direction have served to highlight the importance of a second crucial source of particulates, which is particulates generated internally by the wafers and/or transfer mechanism. That is, when the surface of the wafer jostles slightly against any other hard surface, some particulates (of silicon, silicon dioxide, or other materials) are likely to be released. The density of particulates inside a conventional wafer carrier is typically quite high, due to this source of particulates. Moreover, many of the prior art mechanisms for wafer transport will themselves generate substantial quantities of particulates.
The current state of the art wafer loading mechanisms used in the semiconductor industry consist primarily of three basic types: belt driven wafer transport, air track driven wafer transport, and arm driven wafer transport (using either vacuum coupling or nesting to hold the bottom or the edge of the wafer). However, all of these types of systems typically use face up wafer movement into and out of the carrier, vertical movement of the wafer carrier during the loading and unloading operations, wafer transfer under pressures ranging from atmospheric to low vacuum, and a requirement that wafers be unloaded in the reverse order of loading. The prior art methods accordingly have a number of important disadvantages, as follows.
First, wafers which are transported face up are more likely to catch particles generated by particle generation mechanisms inside the wafer carrier or inside the wafer loader unit.
Second, vertical movement of the wafer carrier during the loading and unloading operation creates many particles, due to rattling of the wafers in the carrier. These particles can fall directly onto the active face of adjacent wafers resting face up in the carrier.
Third, belt mechanisms will typically scrub the bottom of the wafer during loading and unloading operations, again creating many particualtes due to abrasion.
Fourth, air track transport will stir many particulates around by air currents, and many of these particulates can come to rest on the active face of the wafer.
Fifth, the drive mechanisms of many loader modules are housed within the same area as the open wafer carrier will be, in close proximity to the wafers being processed. This has great potential for gross amounts of contamination.
Sixth, the mass of the carrier wafer combination changes as the wafers are loaded and unloaded, and this can affect the reliability and positioning of the wafer carrier vertical drive, particularly where large wafers (such as 150 millimeters or larger) are being handled.
Seventh, two loading modules are typically used for each processing station, so that one cassette is typically progressively loaded, and the wafers from this cassette which have been processed are loaded into a second cassette.
Eighth, loss of equipment utilization efficiency occurs every time a new cassette of wafers is loaded into or out of each processing station, since the machine must be idled while the cassette is removed.
The present invention provides advantageous solutions to all of the above problems, and achieves greatly improved low particulate wafer handling and loading operations.
One key advantage of the present invention is that wafers can be transported, loaded and unloaded without ever seeing atmospheric or even low vacuum conditions. This is extremely useful, because, at pressures of less than about 10 to the -5 Torr, there will not be enough Brownian motion to support particulates of sizes larger than about 10 nm, and these particulates will fall out of this low-pressure atmosphere relatively rapidly.
FIG. 2 shows the time required for particles of different sizes to fall one meter under atmospheric pressure. Note that, at a pressure of 10 to the -5 Torr (1E-5 Torr) or less, even 10 nm particles will fall one meter per second, and larger particles will fall faster. (Large particles will simply fall ballistically, at the acceleration of gravity.) Thus, an aatmosphere with a pressure below 10 to the -5 Torr means that particles ten nanometers or larger can only be transported ballistically, and are not likely to be transported onto the critical wafer surface by random air currents or Brownian drift.
The relevance of this curve to the present invention is that the present invention is the first to provide a way to transport wafers from one processing station to another, including loading and unloading steps, without ever exposing them to higher pressures than 1E-5 Torr. This means that the wafers are NEVER exposed to airborne particles, from the time they are loaded into the first vacuum processing station (which might well be a scrubbing and pumpdown station) until the time when processing has been completed, except where the processing step itself requires higher pressures (e.g. in conventional photolithography stations or for wet processing steps). This means that the total possibilities for particulate collection on the wafers are vastly reduced.
A key element of this advantage is that the present invention provides a method and apparatus for loading and unloading a vacuum carrier under hard vacuum.
The present invention provides a load lock which includes an apparatus for opening a vacuum wafer carrier under vacuum, for removing wafers from the carrier in whatever random-access order is desired, and for passing the wafers one by one through a port into an adjacent processing chamber, such as a plasma etch chamber. Moreover, the load lock of the present invention is able to close and reseal the wafer carrier, so that the load lock itself can be brought up to atmospheric pressure and the wafer carrier removed, without ever breaking the vacuum in the wafer carrier.
A particular advantage of the preferred embodiments of the present invention is that the mechanical apparatus preferably used for wafer transfer is extremely compact. That is, by providing a transfer arm pivoted on an arm support, with gearing or a chain drive inside the arm support so that the rotation of the arm support causes twice as much rotation of the transfer arm with respect to the arm support, a compact apparatus is provided which can rest in the home position and require no more clearance than the length of the arm support in one direction, but can be extended, by a simple rotary shaft motion, out to the length of the arm support plus the length of the transfer arm in either of two directions.
A further advantage of the preferred embodiments of the present invention is that the motors used to extend the transfer arm and to change its elevation are both held inside an exhaust manifold, so that particulates generated by these moving mechanical elements do not tend to reach the interior of the load lock chamber where wafers are exposed.
A further advantage of the invention is that a transfer arm is provided which can handle wafers face down with minimal damage to device areas caused by contact with the transfer arm.
A further advantage of the present invention is that the present invention provides a wafer transfer apparatus which can handle wafers with minimum generation of particulates caused by the handling operation.
A further advantage of the present invention is that the present invention provides a transfer apparatus which can handle wafers with essentially no generation of particulates due to abrasion, since essentially no sliding contacts are made.
Another advantage of the wafer transport mechanism of the present invention is that control is simplified. That is, the transfer arm preferably used has only two degrees of freedom, and position registration is provided so that the transfer arm control can be provided very simply (by use of stepper motors or comparable apparatus), without the need for sensors to detect the position of or forces on the arm.
A related advantage of the wafer transport mechanism of the present invention is that it is a stable mechanical system. That is, small errors in positioning do not accumulate, but are damped out by inherent negative feedback provided by some of the mechanical elements used. This also facilitates the advantage of simple control.
A further advantage of the present invention is that the wafer handling equipment used in the load lock takes up minimum volume. Since the load lock is of such small volume, vacuum cycling can be performed rapidly without requiring very expensive large vacuum pumps.
An even more important consequence of the volume efficiency of wafer transport according to the present invention is that the upper portion of the load lock (wherein the defect-sensitive surfaces of wafers being transferred will be exposed) will therefore have a small surface area. It is desirable to have as little surface area as possible within line-of-sight of the wafer surface, and it is also desirable to have as little surface area as possible in close proximity to the wafer surface, whether it is within line-of-sight or not. All surface area in the upper part of the load lock (i.e. the part above the exhaust manifold) presents two hazards: first, all surface area will desorb gases, so that the more surface area is in the upper chamber the more difficult it will be to pull a hard vacuum. Second and more important, all surface area has the potential to hold adherent particulates, which can later be expelled by mechanical vibration or shock to fly ballistically onto the wafer surface, even under high vacuum. Thus, the volumetric efficiency of the load lock according to the present invention means that the potential for ballistic transport of particulates onto the wafer surface is reduced.
Another advantage resulting from the compactness of the wafer handling equipment in a load lock according to the present invention is that clean room floor space (which is very expensive) is not excessively consumed by such an apparatus.
Another advantage of the wafer carrier described in the present patent application is that this wafer carrier cannot inadvertently be opened outside a clean room. A substantial yield problem in conventional clean room processing is inadvertent or careless exposure of wafers to particulates by opening the wafer carrier outside the clean room environment. However, with the wafer carrier of the present invention this is inherently impossible, since the pressure differential on the door of the carrier holds it firmly shut except when the carrier is in vacuum. This is another reason why the present invention is advantageous in permitting easy transport and storage of wafers outside a clean room environment.
In a further class of embodiments of the present invention, a process module (which may optionally contain one process station or more than one process station) has more than one load lock according to the present invention attached to it. Thus, processing can continue on wafers transferred in from one load lcok while the other load lock is being reloaded. Moreover, the provision of two transfer mechanisms means that, if a mechanical problem occurs with one transfer apparatus inside its load lock, the processing station can continue in production, using transfer through the other load lock, while a technician is summoned to correct the mechanical malfunction. Thus, this class of embodiments has the advantage of greater throughput.
In a further class of embodiments of the present invention, a process module which contains more than one sealable process station incorporates a load lock as described, together with wafer handling equipment inside the process module to transfer wafers received from the load lock wafer handling equipment to any selected one of the process stations. This has the advantage that high lot throughput can be achieved while using single-slice processing techniques and low-particulate high-vacuum handling.
The foregoing arrangement can be used with a great variety of processing modules. However, one particular embodiment, which not only exemplifies generally applicable features of such modules (and the advantages derived therefrom) but also contains numerous specific innovative plasma reactor features (which are also applicable to reactors using more conventional wafer transport, but are particularly advantageous for use with this load lock and wafer transfer arrangement) will be discussed in detail. This sample embodiment is a plasma etching station.
A common problem with prior art plasma etching machines has been particulate generation. Typically plasma etching stations are among the worst stations for deposition of particulates on the sensitive active area of the wafers. One particularly difficult area is the use of "shower head" gas distribution systems, where gas is distributed to the face of the wafer through multiple holes in the face of the active electrode. While this arrangement would seem to to be an attractive way to provide a highly uniform distribution of available process gasses over the face of the wafer, in practice, polymeric or other byproducts of the plasma phase reactions tend to deposit in the holes in the shower head, providing particulates which can be blown through and then immediately deposited on the wafer.
A novel teaching of the present invention is that the flow of gasses to the wafer face should be dominated by diffusion. That is, this aspect of the present invention provides a low-pressure plasma etching (or reactive ion etching) station wherein there is no bulk gas flow over the wafer face. that is, the feed gas species (which are needed for the plasma reactions which will produce the desired ions and free radicals to actually effect etching) are transported into the high-field region close to the face of the plasma (where they will be available for dissociation), not by bulk flow of a gas stream, nor (preferably) even by turbulent eddy currents in a region where there is no overall average bulk flow, but by diffusion. this means that particulate transport by gas-flow-assisted transport to the face of the wafer is greatly reduced.
In a further feature of this plasma reactor, the feed gas distributor is made of an insulating material. The feed gas distributor can thus be positioned reasonably close to the wafer face (i.e. less than one wafer diameter away) to help assure uniform availability of the reaction gasses, but there will not be large potential drop across the dark zone in the plasma immediately adjacent to the feed gas distributor (as there would be if the distributor were made of a conductive material), and thus deposition of polymeric or other plasma reaction products on the feed gas distributor will be reduced. This means that transport of these reaction products to the wafer face as particualtes will also be reduced.
In a further feature of the invention, wafers are etched in a face down position, and a gas distributor is provided which is below the wafer face and has ports blowing away from the wafer. This helps to assure that the bulk gas flow is downward and away from the wafer face, and thus reduces the likelihood of transport of particulates to the wafer face.
A further innovative feature taught by the present application is a plasma (or RIE) reactor for face down etching of wafers, wherein essentially all of the grounded metal reactor chamber walls seen by the plasma move as a unit to open and close the reactor. That is, a vacuum bellows is provided which supports the counter electrode opposite the wafer, and also supports the chamber sidewalls and (preferably) the process gas distribution distributor, so that all of these elements move as a unit. By reducing mechanical movements in proximity to the wafer, generation of particulates thereby is reduced.
The prior art of openable and closeable RIE reactors would typically use complex mechanical actuators, such as feedthroughs or cams, to clamp the wafers in place. However, in the present invention, the only moving mechanical elements are the wafer support pins, which are mounted on (and move only a short distance against the pressure of) an elastic support, so that the process chamber bottom portions can be closed against the powered electrode support when the plasma reactor is sealed to begin plasma etching.
Another innovative feature of this embodiment is that a quartz top layer is provided on the chamber housing, as the layer which will mate to the powered electrode support. This quartz top layer helps to preserve a high area ratio of powered electrode area to ground plane area in the chamber, which provides enhanced ion bombardment on the powered electrode, which is well known to those skilled in the art. This enhanced bombardment is desirable to assist in anisotropic wafer etching. The use of quartz here is further advantageous in that it is transparent to a wide variety of ultraviolet wavelengths, so that optical end point detection, and operator inspection of the plasma etching operation, are both facilitated.
The quartz top layer is also configured to provide excellent uniformity of ion bombardment on the surface of the wafer. That is, the plasma is confined by quartz walls, for several centimeters away from the face of the wafer, to the shape of a cylinder having approximately the same width as the wafer. This collimation of the plasma provides improved uniformity. Quartz walls are particularly well suited for this collimation, since they have less interaction with the plasma than metal walls would.
A further innovative feature of this embodiment is that a light bleed of helium is preferably provided to the back side of the wafer. This helium bleed assures good and uniform thermal contact between the wafer and the powered electrode under vacuum.
According to the present invention there is provided: A wafer processing module comprising: a vacuum-tight processing module chamber; a plurality of process stations inside said module chamber; at least one wafer stage inside said module chamber; a module transfer arm inside said module chamber, said module transfer arm being controllable to transfer wafers to and from said wafer stage and to and from a plurality of said process stations; and at least one load lock abutting said module chamber, said load lock having space therein to hold a wafer carrier, said load lock having a door opener therein, said door opener being controllable to open the door of one of said wafer carriers inside said load lock while said load lock is under vacuum, said load lock being connected to a vacuum pump capable of pulling a vacuum higher than 10 to the -3rd Torr, said load lock also including a transfer arm controllable to remove a desired wafer from the wafer carrier and place the desired wafer on said wafer stage.
According to the present invention there is also provided: A method for fabricating integrated circuits, comprising the steps of: providing a plurality of wafers in a vacuum sealable wafer carrier box; placing said wafer carrier box into a vacuum sealable load lock attached to a process module; pumping down said load lock to a pressure less than 10 to the -4 Torr; opening said wafer carrier and extending a load lock transfer arm into said wafer carrier, to remove a selected one of said wafers therefrom; transferring wafers in a desired sequence from said wafer carrier to one or more selected process stations inside said process module and back until a desired sequence of processing operations has been completed; and then closing said wafer carrier and raising the pressure of said load lock to approximately atmospheric, so that said wafers remain in vacuum inside said wafer carrier while said door of said wafer carrier is held closed by differential pressure.
According to the present invention there is provided: A method for fabricating integrated circuits, comprising the steps of: providing a plurality of wafers in a wafer carrier box, said wafer carrier box having a vacuum sealable door; placing said wafer carrier box into a load lock attached to a process module, said load lock having a vacuum sealable lid; closing said lid and pumping down said load lock to a pressure less than 10 to the -4 Torr; opening said door of said wafer carrier and extending a load lock transfer arm into said wafer carrier, to remove a selected one of said wafers therefrom; transferring wafers in a desired sequence from said wafer carrier to one or more selected process stations inside a process module and back until a desired sequence of processing operations has been completed by selectively elevating and extending said load lock transfer arm toward said wafer carrier to remove or replace wafers from selected slots in said wafer carrier, and away from said wafer carrier, through a port between said load lock and said process module, to transfer wafers between said wafer carrier and a wafer stage inside said process module, and by selectively elevating, extending, and rotating a module transfer arm to selectively transfer wafers among said wafer stage and said process stations; and closing said door of said wafer carrier, closing said port in said adjacent processing chamber, and raising the pressure of said load lock to approximately atmospheric, so that said wafers remain in vacuum inside said wafer carrier and said door of said wafer carrier is held closed by differential pressure; and removing said wafer carrier from said load lock.